1. Field of the Invention
The present invention relates generally to the field of organic thin films. More particularly, the present invention relates to organic monolayers that are formed via an addition reaction. Specifically, a preferred embodiment of the present invention relates to an organic monolayer that is formed on a silicon (001) surface with the Si.dbd.Si dimers of the surface acting as a template for extending the translational and rotational order of the surface to the organic monolayer. The present invention thus relates to organic monolayers of the type that can be termed ordered and anisotropic.
2. Discussion of the Related Art
Within this application several publications are referenced by arabic numerals within parentheses. Full citations for these, and other, publications may be found at the end of the specification immediately preceding the claims. The disclosures of all these publications in their entireties are hereby expressly incorporated by reference into the present application for the purposes of indicating the background of the present invention and illustrating the state of the art.
Historically, the (001) surface of silicon has been very important technologically as the starting point for most microelectronic devices. While microelectronics processing typically involves reactions of silicon surfaces with inorganic compounds, there has been increasing interest in developing methods for coupling existing microelectronics technology with organic-based structures for applications such as non-linear optics, thin-film displays, lithography and molecular electronics.
The Si(001) surface is characterized by a surface reconstruction in which adjacent atoms pair together, forming dimers, (i.e., Si.dbd.Si). Since each silicon atom of the bulk-terminated surface has two dangling bonds, the hybridization of two atoms to form a dimer can be described as the formation of a strong .sigma. bond and a weak .pi. bond.
Scanning tunneling microscope (STM) images of Si(001) have revealed filled and empty electronic states having the symmetry properties expected for .pi.-type electronic states as the highest-occupied and lowest-occupied electronic states..sup.(1) The .pi. bonding of Si(001) suggests that the reactions of this surface should in some way be analogous to those of double-bonded compounds of carbon and of silicon..sup.(2)
Despite the great technological importance of the Si(001) surface, there has been comparatively little work attempting to fabricate well-defined organic layers on this surface. Chidsey and co-workers successfully formed layers of densely-packed alkyl monolayers on hydrogen-terminated Si(111) and Si(001) surfaces through a free-radical mechanism..sup.(3) It has been observed previously that ethylene and acetylene are capable of chemisorbing on Si(001) in a so-called "di-.sigma." configuration, in which the double bonds of the Si dimers and the double bonds of the adsorbed organic species are broken, thereby forming two new Si--C .pi. bonds..sup.(4-7) The chemical process responsible for this reaction can be formally termed a "2+2" cycloaddition reaction.
However, a disadvantage of most these previously recognized approaches is a lack of adhesion unless various chemical bonds of the reactants are broken. Typically, these prior art approaches include the breakage of carbon-hydrogen bonds, or even carbon-carbon bonds. The breakage of these bonds introduces disorder into the deposited organic film.